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BIOMECHANICAL EFFECTS OF BALLOON KYPHOPLASTY ON
TREATED AND ADJACENT NON-TREATED VERTEBRAL BODIES:
PRE AND POST OPERATIVE EVALUATION
Muhammad Hazli Mazlan1, Mitsugu Todo2, Hiromitsu Takano3 and Ikuho Yonezawa3
1
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan
Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, Malaysia
2
Research Institute for Applied Mechanics, Kyushu University, Japan3
Department of Orthopedic Surgery, Juntendo University, Japan
E-Mail: hazli.010@s.kyushu-u.ac.jp

1

ABSTRACT
Balloon kyphoplasty (BKP) is one of the most reliable minimally invasive surgery (MIS) procedure to treat
osteoporotic vertebral compression fractures (VCFs). However, untoward complications of high incidence of adjacentvertebral fractures after BKP and their risk factors are still equivocal. To further investigate the underlying cause of thisphenomenon, non-linear finite element analysis (FEA) was fully utilized based on load sharing distribution, strain energy
distribution and fracture risks evaluation. For this purpose, an image-based pre and postoperative three-dimensional (3D)
finite element models of thoracic and lumbar spinal unit (T11-L3) for an osteoporotic patient were developed. After BKP,
the load sharing distribution (between anterior and posterior column), strength and stiffness of the augmented vertebra hassignificantly improved. However, higher generation of interface stresses and deformation energy were found immediately
at the adjacent vertebral bodies, which makes them susceptible to the risks of bone failures. Failure risks evaluation basedon the incremented loads of 1kN to 10kN (50% risk of spinal injury) have shown very encouraging results. Apparently, thepostoperative deformation onset loading (5kN) was fallen within the acceptable range that was higher than the standard
normal daily living activities (1kN). It is believed that, the optimization of the material properties used in BKP procedure
with the depth understanding on the historical and natural evolution of the osteoporosis could achieve optimal clinicaloutcomes in the future.
Keywords: balloon kyphoplasty, vertebral compression fractures, finite element analysis.

INTRODUCTION
Vertebral compression fractures (VCFs) has
become a global issue in relation to the improved
healthcare and lifestyle. The underlying cause of thisdisease might be due to trauma, disease of the spine andthe most common cause is osteoporosis. The clinicalsignificance of osteoporotic lies in its high vulnerability
and susceptibility to bone fractures (Homingga et al.,2004), and the most prevalent fracture site is the spine(Melton and Chrischillies, 1992). It is characterized by the
low bone mass and micro-architectural deterioration of thebone tissues (Peck et al., 1993). It has been reported that
two-thirds of VCFs was detected asymptomatically(Cooper, O’Neill and Silman, 1993) and if left untreated
VCFs would increase the chance of subsequent fracturesby 500% leading to a dramatic health deterioration andperhaps increase the risk of mortality (Mathis et al., 2001).
In Japan, there are more than 10 million osteoporosis
patients (Tawara et al., 2006). It is believed that thisnumber will significantly increase in relation to Japan’slife expectancy continues to rise.
Minimal invasive surgery (MIS) procedure ofballoon kyphoplasty (BKP) is of the most reliable
treatment to treat VCFs disease. BKP requires theinjection of bone cement into the vertebrae in order tostabilize fractured vertebrae, restore the height of thecompressed vertebral body, relieve pain, reduce theincidence of cement leakage into spinal canal, correct

kyphosis deformities, and achieve a fully spinal functional
outcomes (Zhang et al., 2010). Apparently, the selectionof the bone cement as a filling material play the utmostrole in determining the effectiveness of the treatment (Heet al., 2015). Basically, the bone cement must have theconsiderable mechanical strength and toughness, theymust be radiopaque under fluoroscopy, and they shouldhave an adequate viscosity and be injectable forfacilitating BKP procedure (He et al., 2015), (Zhang et al.,2010), (Lavelle and Cheney, 2006). Many reports haveshown clinical improvements for patient treated with BKP(Luo, Adams and Dolan, 2010). However, recurrent
vertebral compression fractures, particularly at theadjacent levels has been raising consternation about its
further complications.
The objectives of this study were to quantitatively
analyze the load transfer mechanisms of pre andpostoperative models of patient with VCFs treated with
BKP procedures. It was also to investigate to what extent
would BKP procedures increase the strength and stiffnessof the treated vertebra and concurrently induce theoccurrence of VCFs at the adjacent non-treated vertebrae
level.
MATERIALS AND METHODS
FE modelingThree-dimensional FE models of pre and6196

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postoperative vertebrae column (T12-L3) of an
osteoporotic patient (Figure-1) were constructed in
MECHANICAL FINDERTM software (Research Center ofComputational Mechanics Co. Ltd. Japan). Written
informed consent, permission and cooperation of 74-yearold Japanese female osteoporotic patient was obtained. To
create the FE models, CT scan images of the osteoporotic
subjects (Department of Orthopedic Surgery, Juntendo
University, Japan) were taken pre and postoperatively andtransferred to FE software. From the obtained CT scan
images, the FE models were then constructed based on theextracted bone edges of the region of interests (ROI)
around the outer region of the cortical bone to obtain the
anatomical structure of the spinal bone. The same process
was also repeated in order to obtain the physical structureof the augmented bone cement in the postoperative model.
The FE models were then modelled with 1mm linear
tetrahedral and triangular elements. The tetrahedral
element was used to represent the bone cement and theinner portion of the cortical and cancellous bone. The
triangular element with the thickness of 0.4mm was used
to represent the outer cortex.
To reflect the heterogeneity of the FE models, themechanical properties for each element were calculated
from the Hounsfield Unit (HU) values. Young’s modulusof each element were obtained using the relationship asreported by Keyak et al. (1998). Poisson’s ratio was set toa constant value of 0.4 (Keyak et al., 1998), (Reilly andBurstein, 1975), (Van Burskik and Ashman, 1981). Facet
joint cartilages and intervertebral discs were created basedon the approximation and visualization of their actual
structure and position that were verified by orthopaedic
surgeons. Poisson’s ratio for the intervertebral discs andthe facet joint cartilages were set at 0.45 and 0.2,
correspondingly. Meanwhile, the young’s modulus wereset at 8.4MPa and 11MPa, respectively (Table-1). The
material properties of the bone cement are listed in Table2.
85%

15%

Bone cement

Fixed
Pre-operative

Post-operative

Figure-1. Pre and postoperative models. The same loads
and boundary conditions were applied to the both model.

Table-1. Facet joint cartilages and intervertebral discmaterial properties.
Material
Intervertebral
Disc
Facet Joint

Young’s Modulus
(MPa)
8.4

Poisson’s
Ratio
0.45

11

0.2

Table-2. Bone cement material properties (Kurtz,
Villarragaa and Zhao, 2005(a)), (Kurtz, Villarragaa andZhao, 2005(b)).
Material properties
Poisson’s ratio
Young’s modulus
Critical strength
Yield strength

Bone cement
3.7
3700MPa
31MPa
111MPaAnalysis
The FE models were loaded with compressive
loading of 1kN (Tawara et al., 2006), (Mazlan et al, 2014).
The load was then incrementally increased up to 10kN inorder to evaluate the failure risks in the vertebrae. The
10kN compressive/impact loading was used to simulate
the worst loading condition, the force at which there is
50% risk of injury (Meyerhoff et al., 2008). The loadings
were applied to the superior surface of T11, while theinferior surface of L3 was fixed in all directions (Figure1). The loads were modeled according to the three-column
load distribution (85% of the load was applied to thevertebral body, 15% to the neural arch) (Zhang et al.,2010).
The biomechanical effects of the pre andpostoperative models were thoroughly analyzed andinvestigated based on the non-linear finite element
analysis. The analysis was focused on the load sharing
distribution at the anterior and posterior column of thevertebrae, and the interface stresses at the intervertebral
disc-endplate and facet joint cartilage junction. In addition,
the strain energy distribution (SED) variations were also
investigated. The SED was obtained from the principle
stress and stress invariant (Chiang and Wang, 2006). It
was reported that the load transmission mechanism of the
SED can be used as a failure criterion of an element.
The fracture risks evaluation of the vertebrae
were analyzed based on the Newton-Raphson method(Bessho et al., 2007), (Tawara et al., 2014). The analysis
were translated into a graphical representation of failure
and yielding element distributions. The criteria of the
failure elements were divided into compressive and tensile
directions. The initiation of the failure in a tensile
direction was initiated when the maximum principle stressis higher than the ultimate tensile stress. The ultimate
tensile stress was set as 80% of the compressive yieldstress. As for the compressive direction, the failure criteria
was divided into two stages according to Drucker-Prager
yield criterion. The first stage known as a yielded state andit occurs when the Drucker-Prager equivalent stress6197

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exceeds the compressive yield stress. The second stage or
the state of the initiation of failure commences when theminimum principle strain of an element is less than -10000
microstrains in a yielded state.
RESULTS AND DISCUSSIONS
Drucker-Prager stress distribution for the pre andpostoperative models under the application of 1kN
compressive loading are shown in Figure-2. In general, theDrucker-Prager stress of the postoperative model was
distributed more evenly than the preoperative model. Thiscondition was closely related to the failure of theintervertebral disc to distribute the compressive stressevenly on the adjacent vertebral bodies preoperatively. By
implementing the BKP procedure, the disc pressure could
be restored back to normal and the stress could be
distributed more evenly on the adjacent vertebral bodies.
Obviously, for the treated L1 vertebra, the highest stressconcentration could be found at the interface between thebone cement and the cancellous bone junction. Thiscondition might be related to the stiffness mismatch
between the bone cement and the cancellous bone, whichmight cause bone resorption in the long run (Mazlan et al.,2014). This condition could be reduced by enhancing thebone cement material properties to the value that is close
to the cancellous bone material properties with improved
mechanical strength and stiffness.
Figure-3 shows the stress distribution between
the anterior (vertebral body) and posterior column (neuralarch) of the vertebrae. Obviously, BKP has significantly
reduced the stress concentration gap between the twosegments especially at the L1 vertebra. The neural archload bearing for the postoperative model at the L1 and L2
experienced lower stress concentration than the preoperative model. The most affected region was at thepedicle region of the L1 vertebra as depicted in Figure-2.
However, the risk of the bone failure was still exist at the
T12 vertebra based on the high stress concentration gap
difference postoperatively. This is based on the fact that,the high stress concentration gap between the neural archand vertebral body might cause the area with lower stressconcentration would be stress shielded, reducing the bonedensity and finally increased the risk of bone failures(Luo, Adams and Dolan, 2010). It has to be borne in mind
that, it is also not good to have a high stress concentrationon the anterior column of the vertebra, since the anterior
column is more susceptible to damage due to its low bonemineral density especially in elderly spine (Lavelle andCheney, 2006).

(a)Pre-operative

Post-operative(b)

Figure-2. Drucker-Prager stress distribution mapped at (a)ordinary and (b) down-sized scale.
Table-3 and Table-4 show the distribution of theMaximal Drucker-Prager stress at disc-endplates and facet
joints junctions, respectively. Obviously, after BKP
surgery, the maximum L1 superior and inferior endplates
Drucker-Prager stresses increased by 27.27% and 96.5%,
respectively. However, at the adjacent non-treatedvertebrae two contradictory results were observed. The
maximum T12 inferior endplate Drucker-Prager stressdecreased by 16.46%, while the maximum L2 superior
endplate increased by 48.98%. The reduction of the
maximal Drucker-Prager stress at the T12 endplate wascompensated with the increased of the stress at the T12
inferior articular facet, while the increment of the maximal
Drucker-Prager stress at the L2 superior endplate wascompensated with the decrease of the stress at the L2superior articular facet. Obviously, after BKP the interface
stresses (inferior and superior endplates) at the treatedvertebra has increased significantly and therefore theadjacent non-treated vertebrae has to absorb the higher
load impacts produced at this region. This showed that thebone cement has increased the total strength and stiffness
level of the treated vertebra and thus increasing the stresson the endplates immediately above and below the treatedvertebral body, leading to increase pressure on theadjacent discs, and eventually producing higher stress on
the endplate of the adjacent vertebrae (Zhang et al., 2010),(Luo, Adams and Dolan, 2010).

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Table-4. Maximal Drucker-Prager interface stress at the
facet joint cartilages junction.

(a)

T12 inferior
articular
facet
L1 superior
articular
facet
L1 inferior
articular
facet
L2 superior
articular
facet

(b)

Figure-3. Graph of Maximal Drucker-Prager stress
distribution at the anterior and posterior column for the (a)pre and (b) postoperative models.
Table-3. Maximal Drucker-Prager interface stress at discendplates junction.

T12
inferior
endplate
L1
superior
endplate
L1
inferior
endplate
L2
superior
endplate

Preoperative

Postoperative

Percentage
difference

12.9

10.8

-16.5%

13.1

16.7

27.3%

8.5

16.7

96.5%

13.5

20.1

49.0%

Preoperative

Postoperative

Percentage
difference

16.5

150.7

811.3%

19.2

76.2

296.9%

314.3

45.4

-85.6%

54.5

43.6-20.0%SED variations as shown in Figure-4 and Figure-5
is one of a reliable method to interpret the load transfer
mechanism and the risk of bone fractures in vertebrae. For
the both cases (pre and postoperative models) the adjacentnon-fractured and non-treated vertebrae experienced
higher SED than the fractured and treated vertebra. Since
the input energy of the motion segment was remained thesame, therefore the adjacent non-treated and non-fractured
vertebrae have to absorb the extra energy and hence
increases the risk of vertebral bone failures. For the
postoperative model, the strain energy density at the L1
has been significantly reduced and the risk of fracture at
this area was assumed reduced. By virtue of the potential
energy stored in a volume, the SED is reflecting theamount of work that has to be done to produce the elastic
deformation (Chiang and Wang, 2006). Therefore, thismechanism could be used as a tool to predict which area ofthe vertebral bodies that are susceptible to vertebral
failures based on this energy type failure criterion (Li,2001). In addition, the results showed that the adjacentnon-treated vertebrae were weaker and less stiff than thosetreated with BKP. This mechanism may explain theclinical observation of the adjacent vertebral failure after
BKP.

Pre-operative

Post-operative

Figure-4. Strain energy density distribution.

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Figure-5. Graph of strain energy density distribution at theanterior and posterior column.
To further enhance the investigation on thephenomenon of the adjacent non-treated vertebral
fractures, bone fracture analysis based on the NewtonRaphson method was applied. The distribution of the
failure and yielding shell elements in tensile andcompressive directions under the incremented compressive
loading of 1kN to 10kN are given in Figure-6. Apparently,
the initiation of element failure was started to show its
significant impact when the load was increased from 7kN
to 10kN. The amount of failure and yielding shellelements at the adjacent non-treated vertebrae T12 and L2
were comparably higher for the postoperative than the
preoperative model. However, at the treated vertebra L1
the number of the yielding and failure elements has
decreased postoperatively as described in Figure-6(b).
These phenomena were believed to have a strong
relationship with the improved mechanical properties of
the treated vertebra. As a result, the fractures were
initiated at the adjacent vertebral bodies. This situation
could be translated into a graphical representation as
depicted in Figure-7. The fracture of the vertebrae were
defined as occurring when at least one shell element
failed. Therefore, it was comprehensively understood thatthese results will be highly reflected its clinical outcomesin a real situation (Bessho et al. 2007).

Figure-6. Historical changes in the number of shellelements having undergone tensile failure, compressive
yielding and compressive failure under the application ofincremented loading of 1kN to 10kN.
(a)

(b)

(a)

Figure-7. Distribution of failure and yielding shellelements under the application of 10kN (a) pre and (b)postoperative models.
The Finite element analysis have shown a verysignificant impact on the evaluation of BKP procedures.
The adverse effect of the VCFs in osteoporotic and elderly
people is well understood (Uppin et al. 2003). Spinal
deformity biomechanically produced uneven high stressand strain distributions leading to the initiation ofsubsequent fractures even with a normal daily living

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activities (Mazlan et al. 2014). Postoperative results has
shown that BKP has significantly improved the load
sharing distributions between the anterior and the posteriorvertebral coulmn and tremendously increased the stiffness
and strength of the fractured vertebral body towards
prefracture levels. Moreover, the onset load for theinitation of the failure and yielding elements at the treatedvertebral level of L1 was significantly improved incomparison to the preoperative model. As for the fractured
vertebra, the onset load could be detected as early as 5kN,
whereas for the treated vertebra only microdamages weredetected at the maximum load of 10kN. Generally, after
BKP surgery, bone damage formation was started to occur
under the impact loading of 7kN. This condition ismechanically acceptable since the onset load was much
more higher than the normal daily living activities asreported by Han et al.2013. However, the bone failurerisks was still exist at higher load and therefore high risks
activity must be completely avoided. Inevitable events
such as falling or accidents might still have imposed highsusceptibility of bone failure either on the treated or nontreated vertebrae.
Despite its superior characteristic in improving themechanical properties of the fratured vertebra, the result
has suggested that BKP could increase the stress on theendpates immediately above and below it, and hence
producing higher stress on the adjacent vertebral
endplates, which then putting them in a state of bone
failures. It has also shown that the results of the SED loadtransfer mechanisms (Chiang and Wang, 2006) was
correlated well with the results of the failure analysis
utilizing by Newton-Raphson method (Bessho et al. 2007),(Tawara et al. 2014). Eventhough the main contributing
factors of this phenomenon is still equivocal, we arestrongly believed that a few factors like the type andamount of the injected bone cement, bone loss and natural
evolution of osteoporosis on the spine were underlying
the cause of this problem (Uppin et al. 2003) (Luo, Adams
and Dolan, 2010), (Zhang et al. 2010), (He et al. 2011)(He et al. 2015). In a related study, bone cement leakage
into veretebral disc was detected as one of theconfounding factors that increases the risk of a new
fracture at the adjacent vertebral bodies (Lin et al. 2004).
In addition, rapid clinical improvement postoperatively
might cause the patient to be able to get involve in more
active activities that they were unable to do it previously.
Eventhough they may resume fully recovery, the new axial
load on the veretbra may be stressful and leading to a newcompression fractures on the adjacent vertebrae (Uppin etal. 2003).
Improving mechanical properties of the bone
cement has been one of the main focus in optimizing theusage of the materials in BKP development.
Biomechanically, the bone cement must be have anadequate stiffness, compressive, shear and tensile strength
in order to avoid massive biomechanical alteration of thespine (Zhang et al. 2010), (He et al. 2015). Besides, thebone cement choices must be based on comprehensive
mechanical consideration to be able to be well adapted to

the natural environment of the spinal bone (Verlan, Onerand Dhert, 2006). Moreover, being able to simultaneuosly
promote bone formation yet suppress bone resorption is anadded advantage for a promising type of bone cement
utilizes in BKP procedure (He et al. 2015). Thus, therecurrent VCFs at the adjacent non-treated vertebrae could
be highly avoided in the future.
CONCLUSIONSFEA studies have shown that BKP procedure on
the osteoporotic patient has significantly improved thebiomechanical outcomes of the treated and non-treatedvertebral bodies. Moreover, the postoperative deformation
onset loading was fallen within the acceptable range thatwas higher than the normal daily living activities. The
phenomenon of high incidence of adjacent vertebral bone
failures could be well understood and showed very
encouraging results. Therefore, the complimentarymechanisms of the stress distribution, SED distribution
and non-linear fracture analysis could be used as reliable
tools to predict and investigate the effectiveness of thegiven surgical procedure.
ACKNOWLEDGEMENT
This research was partially supported by The
Ministry of Education Malaysia.
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